The present invention relates to the field of fast neutron spectroscopy, with applications in neutron dosimetry, nuclear physics, nuclear defence and counter-terrorism, and other neutron-related research.
The detection of neutrons is one of the most challenging tasks in radiation detection. There are very few physical processes that enable the detection of neutrons, especially fast neutrons (i.e. neutrons with energies greater than approximately 0.5 MeV).
Traditional Detection of Fast Neutrons
Direct fast neutron detection has traditionally relied mainly upon hydrogen recoil (the (n, p) reaction), where the neutron (n) strikes a proton (p), and the proton recoils from the reaction in a similar way that a billiard ball recoils when struck by the cue ball. Depending on the direction of recoil, the energy of the proton can range from zero (glancing hit) to the entire energy of the incident neutron (direct hit). It is the measurement of the energy of the charged proton that is used to infer the presence and energy of the uncharged neutron. Since the proton energy spans from zero to the actual neutron energy, a monoenergetic neutron produces a “distribution” of proton energies and “spectral unfolding” is required to construct a monoenergetic neutron peak (see C. D. Swartz and G. E. Owen, Fast Neutron Physics, Part 1, ed. J. B. Marion and J. L. Fowler (Interscience, New York, 1960) 211-246).
The hydrogen that provides the proton for neutron detection can be present in gases, liquids or solids. Hydrogen in a gaseous medium is usually deployed in “gas counters”, where ionization produced by the recoiling proton is collected by an anode (under high voltage) to produce an electronic signal proportional to the proton energy. Hydrogen in a liquid medium is usually a component of a scintillation cocktail, where the energy of the recoiling proton is converted into a light pulse. The light pulse is then detected by a photomultiplier or other photon detector which gives an electronic signal proportional to the proton energy. Hydrogen in a solid medium is often used as a proton radiator. The function of the radiator is to transfer kinetic energy from a neutron to a proton ((n, p) reaction) so that the latter can be detected by a charged particle detector, such as a silicon diode or a scintillator. The output of the charged particle detector is an electronic signal whose size is proportional to the proton energy. Hydrogen can also be present in a solid scintillation crystal. The recoiling protons produce light in the scintillator and the light is converted to an electronic signal through the use of a photomultiplier or other photon detector.
In all of these detection processes, electronic processing of the signals is required to yield reasonably-sized and appropriately shaped pulses for counting by electronic scalers or for pulse-height analysis for performing neutron spectroscopy. Computer analysis is required to generate the incident neutron energy spectrum. More details on the various methods are given by Cross and Ing (W. G. Cross and H. Ing, The Dosimetry of Ionizing Radiation, Vol. 11, ed K. R. Kase, B. E. Bjarngard and F. H. Attex (Academic Press, New York, 1987) 91-167).
Traditional Detection of Thermal Neutrons
Sources of neutrons always produce fast neutrons. Such sources include nuclear fission (e.g. reactors), high-energy accelerators (e.g. medical linear accelerators or high-energy particle accelerators), isotopic neutron sources (e.g. 252Cf, alpha-n sources, gamma-n sources) and space radiation (e.g. cosmic rays and solar radiation). When fast neutrons interact with matter, they scatter and lose energy in the process. After a few scatters, the fast neutrons become “thermalized” in the sense that their motion is comparable to the normal motion of molecules at room temperature. For many applications, it is convenient to define thermal neutrons as those whose energy is below approximately 1 eV.
There are several isotopes that have affinity for thermal neutrons. Of particular interest are 6Li and 10B. These isotopes have very large absorption cross-sections for thermal neutrons and their respective reactions give rise to energetic alpha particles, whose detection is similar to and simpler than the detection of protons from hydrogen recoil. However, these isotopes are normally used merely for the detection of thermal neutrons, rather than for fast neutron spectroscopy. The reason is that their cross-sections for fast neutrons are several orders of magnitude smaller than for thermal neutrons, leading to much poorer detection efficiency.
The development of crystals containing 6Li or 10B for detection of thermal neutrons is the objective of many groups involved in the art of crystal growing for radiation detection applications. Combes et al (Journal of Luminescence 82 (1999) 299-305) prepared and studied the optical and scintillation properties of pure and Ce3+-doped Cs2LiYCl6 crystals. Koroleva et al (Nucl. Instr. Meth. 537 (2005) 415-423) reviewed various scintillators for neutron measurements. R. Boutchko et al reported on work on the scintillation properties of Cerium activation in Lanthanum and Yttrium Aluminum Perovekites (Sym, L., Nucl. Radiat. Detect. Mat., Apr. 14-16(2009)). J. Glodo et al (2009 IEEE Nucl. Sci. Sym. Conf Record N17-6) reported on the use of Cs2LiYCl6: Ce for the detection of thermal neutrons.